题目

若A为m×n阶矩阵，AB=C，则B的阶数可以是下列中的 ．①m×m，②m×n，③n×m，④n×n． 答案：【1】③④ --- Hi, Tracy, you look tired. --- I’m tired. I ______ the living room all day. A. painted         B. had painted       C. have been painting     D. have pained